Composite structural member

ABSTRACT

A composite structural panel is provided. The panel comprises a frame having a longitudinal frame axis; a plurality of beam elements mounted in the frame, each of the beam elements has a respective longitudinal beam axis generally parallel to the frame axis; a reinforcing fibre wrapping that extends around each of the beam elements, the wrapping has a fibre direction running generally obliquely to the beam axis; a plurality of wire strands that extend about the wrapping, the wire strands run generally parallel to the beam axis and the frame axis; an outer layer of reinforcing fibre mat that surround the frame, the wrapped beams and the wire strands, the mat has a majority of its fibres running generally parallel to the frame axis; and, a solidified epoxy resin that extends throughout and encasing the beam elements, beam wrapping, wire strands, outer layer and frame.

FIELD OF THE INVENTION

[0001] The invention relates to a structural member, generally. In particular, the invention relates to a structural member constructed of composite materials.

BACKGROUND OF THE INVENTION

[0002] Structural members (e.g., columns or beams) may be constructed of a variety of building materials, such as concrete, wood or steel, each of which has particularly advantageous or disadvantageous structural and load bearing properties. For example, concrete alone has fairly good compressive strength however its tensile and bending strength can be improved greatly with the use of reinforcing materials such as hoop steel. Without any reinforcement, however, transverse and axial loads may cause a concrete support structure to spall and ultimately fail.

[0003] Wood is often used in structural applications because it too has good load bearing properties. Wood may be described as an orthotropic material. It has unique and independent mechanical properties in the directions of three mutually perpendicular axes (longitudinal, radial and tangential).

[0004] When a wooden beam is subjected to a load that is perpendicular to the longitudinal direction of the wood body, the wood fibers opposite the load are subjected to tension forces and wood fibers adjacent the load are subjected to compression forces. Under a heavy load, a wood beam may fail in either the compressive or tensile half of the wood beam.

[0005] There are drawbacks to using non-reinforced wood as a building material. For example, wood deteriorates under certain conditions, such as a high moisture or wet environment. Wood is primarily composed of cellulose, lignin, hemicelluloses and minor amounts of extraneous materials. Cellulose is a high-molecular-weight linear polymer consisting of chains of glucose monomers. The cellulose molecules are arranged in ordered strands, which in turn are organized into the larger structural elements that make up the cell wall of wood fibers. Lignin is concentrated toward the outside and between the cells. It is the cementing agent that binds individual cells. It is a three-dimensional phenyl-propanol polymer. The hemicelluloses are associated with cellulose and are branched, low molecular weight polymers composed of several different kinds of pentose and hexose sugar monomers. As the cellulose, lignan, hemicelluloses structure deteriorates, the wood fiber strength is compromised and as a result, so are its load bearing capabilities. Under this weakened condition, the wood is more prone to failure under load. Also, despite any environmental deterioration, wood, under a high load may still experience compression or tension failure if it is not reinforced.

[0006] Previous attempts to reinforce concrete, wood or steel support structures (e.g., Michalcewiz U.S. Pat. No. 5,505,030) involve using pre-made reinforcing layers which are attached or fitted to the element being reinforced, thereby increasing the reinforced element's compressive, shear or load bearing capacity. Solutions of this type, however, are merely prophylactic because they do not address the underlying problem of using a building material that is susceptible to load failure. The effective use of casing reinforcing materials, such as described by Michalcewiz, is further limited by the fact that they are merely surface treatments. It may not be possible to apply reinforcing materials to parts of a structure that are not readily accessible.

[0007] There remains a need for a composite structural member with increased resistance to failure under load.

SUMMARY OF THE INVENTION

[0008] The present invention provides a structural panel comprising a frame having a longitudinal frame axis; a plurality of beam elements mounted in the frame, each of the beam elements has a respective longitudinal beam axis generally parallel to the frame axis; a reinforcing fibre wrapping that extends around each of the beam elements, the wrapping has a fibre direction running generally obliquely to the beam axis; a plurality of wire strands that extend about the wrapping, the wire strands run generally parallel to the beam axis and the frame axis; an outer layer of reinforcing fibre mat that surround the frame, the wrapped beams and the wire strands, the mat has a majority of its fibres running generally parallel to the frame axis; and, a solidified epoxy resin that extends throughout and encasing the beam elements, beam wrapping, wire strands, outer layer and frame.

[0009] The reinforcing fibre mat of the beam wrapping may be a woven mat having about the same quantity of fibres extending in each of two generally orthogonal directions.

[0010] The beams may be made up of a plurality of panels having adjacent faces laminated together and having a grain running along the beam axis.

[0011] The structural panel may be a bridge deck having an upper face coated with a road surfacing material.

[0012] The upper face may be canted with a longitudinally extending centre section higher than and sloping toward opposite side edges of the structural panel to promote drainage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Preferred embodiments of the present invention are described below with reference to the accompanying drawings in which:

[0014]FIG. 1 is a schematic view illustrating a composite structural member according to the present invention;

[0015]FIG. 2 is a schematic view illustrating a beam according to the present invention and the beam's grain orientation relative to its longitudinal axis;

[0016]FIGS. 3a-b are schematic views illustrating alternate orientations of a fibrous material in sheet form and its orientation as it extends about and between a plurality of beams according to the present invention;

[0017]FIGS. 4a-b are schematic views illustrating the use of a fibrous material in cord form and its orientation as it extends about and between a plurality of beams according to the present invention;

[0018]FIGS. 5a-b are schematic views illustrating the use of a fibrous material in both sheet and cord form in a structure according to the present invention.

[0019] FIGS. 6 is a perspective cross-sectional view of a composite structural member according to an embodiment of the present invention;

[0020]FIG. 7 is a cross-sectional view of a bridge and its associated components assembled using a composite structural member according to an embodiment of the present invention;

[0021]FIG. 8(a) is a schematic plan view of a structural panel according to an alternate embodiment of the present invention;

[0022]FIG. 8(b) is a schematic cross-sectional view of the structural panel of FIG. 8(a);

[0023]FIG. 9 is a plan view of a frame element of the structural panel of FIG. 8 (a) according to the present invention;

[0024]FIG. 10(a) is a longitudinal cross-sectional view of the frame of FIG. 9 according to an embodiment of the present invention;

[0025]FIG. 10(b) is a longitudinal cross-sectional view of the frame of FIG. 9 according to an alternate embodiment of the present invention;

[0026]FIG. 10(c) is a cross-sectional view of the frame of FIG. 9 according to an embodiment of the present invention;

[0027]FIG. 11 is a partial plan view of an end portion of a beam element of the structural panel of FIG. 8 according to an embodiment of the present invention;

[0028]FIG. 12 is a perspective view of an end portion of a beam element of the structural panel of FIG. 8 according to an embodiment of the present invention;

[0029]FIG. 13 is a perspective cross-sectional view of a bridge deck according to an embodiment of the present invention;

[0030]FIG. 14 is a perspective view of a beam element of the structural panel of FIG. 8 according to an embodiment of the present invention;

[0031]FIG. 15 is an exploded view of the constituent components of the structural panel of FIG. 8 according to an embodiment of the present invention; and,

[0032]FIG. 16 is a perspective cross-sectional view of the structural panel of FIG. 8 according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0033]FIG. 1 illustrates a composite structural member (CSM) 10 made in accordance with a preferred embodiment of the invention.

[0034] Referring to FIGS. 1-2 and 6, the CSM 10 has a plurality of beams 20, each beam having a longitudinal axis 22 and at least one side 24 generally parallel to and facing at least part of a side 26 of at least one adjacent beam 20. A fibrous material 30 extends about and between the beams 20. A potting material 40 permeates the fibrous material 30 and encases the beams 20. The fibrous material 30 and potting material 40 are selected to yield a resistance to bending in the CSM 10 that is greater than the combined individual resistance to bending of the beams 20.

[0035] Each beam 20 has a grain direction 28 that is generally aligned with the longitudinal axis 22. and is preferably a member selected from the group consisting of wood and wood composites. In an alternate embodiment, the beams 20 may be constructed of concrete, fiberglass, and possibly other materials with properties similar to a wooden beam.

[0036] In a preferred embodiment, the surfaces of the wooden beams 20 and the fibrous material 30 should be treated to clean and prepare the surfaces for bonding. To enhance the bonding between the wooden beams 20 and the fibrous material 30, the wooden beams should be dry with their surfaces relatively free from grease, excessive dust, etc. Preferably the surfaces should be rough to enhance adhesion. A water proofing treatment might also be considered, particularly where the CSM 10 is to be used in a submersed or partially submersed environment and there is a risk of the encapsulating material being damaged so as to expose the wooden beams 20 to water.

[0037] Referring to FIGS. 3-5, the fibrous material 30 is constructed of engineering materials having a high tensile strength. In alternate embodiments, a fibrous material 30 having higher or lower tensile strength and modulus properties may be used depending on the particular application of the invention.

[0038] Referring to FIG. 5a, an embodiment of the fibrous material 30 is in sheet form 32. In an alternate embodiment depicted in FIG. 5b, the fibrous material is in cord form 36. The fibrous material 30 in the sheet form 32 may have variety of fiber architectures. In a preferred embodiment, the fibers 34 are woven. Alternately, the fibers 34 may be braided or knitted. The majority of the fibers 34 in the sheet form 32 are preferably unidirectional in the longitudinal direction of the sheet form.

[0039] Referring now to FIG. 3a, the fibers 34 of the sheet form 32 are preferably aligned generally transversely (i.e. non-parallel) relative to the longitudinal axis 22 of the beams 20. FIG. 3b illustrates an alternate embodiment where the fibers 34 of the sheet form 32 are aligned generally diagonally relative to the longitudinal axis 22 of the beams 20.

[0040]FIG. 4a illustrates a further embodiment. The fibers 34 of the cord form 36 run generally transversely relative to the longitudinal axis 22 of the beams 20. FIG. 4b illustrates a still further embodiment where the fibers 34 of the cord form 36 run generally diagonally relative to the longitudinal axis 22 of the beams 20. FIG. 6 depicts a still further embodiment wherein the fibrous material 30 extends around an individual beam 20, but does not extend around any adjacent beams 20.

[0041] In a preferred embodiment of the invention, the fibrous material is a member selected from the group consisting of glass, carbon, Kevlar™, aramids, nylon, possibly natural fibers (e.g. hemp) and combinations of the foregoing.

[0042] Referring now to FIG. 1, the potting material 40 is a curable resin such as vinyl esters, poly esters, urethanes, BMIs, phenolics, acrylics, epoxies, cynate esters and thermoplastics. The potting material is preferably an epoxy that has exceptional adherence to steel, such as the Jeffco 4101-08 epoxy resin and Jeffco 4101-18 epoxy hardener as manufactured by Jeffco Ltd. of San Diego, Calif.

[0043] The term “resin” refers to any substance, or combination of substances, of a suitable viscosity, such that they can be used to impregnate the fibrous materials and surround the plurality of beams and ultimately undergo a physical state transformation from a low viscosity fluid to a rigid solid state wherein said transformation can occur via various means such as chemical reactions, a thermal cycle, etc. and acts as a binding matrix of the fibrous material and beams to create a final composite material.

[0044] A “Vacuum Assisted Resin Transfer Method” (VART) may be utilized to encase the beam/fibrous material structure in resin. This is a known method of impregnating fibers with resin.

[0045] Sample Applications

[0046] Bridge construction is a sample application of the CSM 10 of the present invention that illustrates its unique and enhanced load bearing properties over prior generation composite structural materials.

[0047] Referring to FIG. 7, a bridge 70 consisting of a substructure and a superstructure is illustrated. Substructure elements include abutments 72, piers or pilings. Superstructure elements, which sit atop the substructure, include stringers and or a deck 74. These structural elements experience compression and tension forces from several potential sources. For example, an abutment 72, pier or piling, would experience compression forces 76 from the weight of the superstructure, the weight of any body positioned on the superstructure and the weight of the abutment 72 pier or piling itself. Superstructure elements, such as the decking 74, may experience compression forces 77 and tension forces 78 from the weight of the decking 74 itself as well as any body 79 sitting on the decking. The combined compression and tension forces may cause the bridge structural elements to buckle (in the areas experiencing compression forces) and or snap (in areas experiencing tension).

[0048] A composite structural member of the type described in the present invention dissipates the compression and tension forces over the whole of the structural member, unlike individual structural members, such as wooden beams or glue-laminated timber (glulam). This load dissipating property of the CSM 10 permits the construction of bridges of greater scale and enhanced load-bearing properties. This is because the load-bearing versus weight ratio of the composite structural member of the present invention is several times that of concrete or steel.

[0049] Bridge construction using the CSM 10 creates an integrated structure where the decking is self-supporting, rather than simply used as cladding. The structural strength of the CSM 10 stems from the fact that the combined strength of the CSM 10 components exceeds that of the beams and the potted encasement elements on their own, in the configuration in which they are present.

[0050] Alternate Embodiment

[0051] Referring to FIGS. 8(a)-(b), 15 and 16, a composite structural member according to a preferred embodiment of the present invention is illustrated. The composite structural member or structural panel 100 is comprised of a frame 101 having a longitudinal frame axis 103. A plurality of beam elements 105 are mounted in the frame 101, each of the beam elements 105 has a respective longitudinal beam axis 107 that is generally parallel to the frame axis 103. The beam elements 105 are wrapped in a beam wrapping 109 of reinforcing fibre mat that extends around each of the beam elements 105, the beam wrapping 109 has a fibre direction running generally on a bias with (or obliquely to) the beam axis 107. The structural panel 100 further includes a plurality of wire strands 111 that extend about the wrapping 109. The wire strands 111 run generally parallel to the beam axis 107 and the frame axis 103. There may also be transverse wire strands 151. The frame 101, wrapping 109, beams 105 and wire strands 111 are further wrapped and surrounded by an outer layer of reinforcing fibre mat 113 that has a majority of its fibres 115 extending longitudinally generally parallel to the frame axis 103. A solidified epoxy resin 117 extends throughout and encases the beam elements 105, beam wrapping 109, wire strands 111, outer layer 113 and frame 101.

[0052] Referring to FIG. 9, the frame 101 defines and determines the approximate size and shape of the structural panel 100. In a preferred embodiment, the frame 101 is comprised of two side members 119 positioned opposite each other and running generally parallel to the longitudinal frame axis 103. The side members 119 are connected to each other via two end members 121, which are positioned opposite each other and generally orthogonal to the side members 119. In an alternate embodiment, the frame 101 is a unitary structure. Any other frame configuration known to those skilled in the art that defines and determines the approximate size and shape of the structural panel 100 may be employed.

[0053] The frame 101 may be comprised of steel. In a preferred embodiment, the frame 101 is comprised of a mild steel, such as a 44W High Strength Low Alloy steel, although any standard grade steel that can be flame cut, formed, drilled, welded and/or machined by any normal means may be employed.

[0054] Referring to FIG. 10(a), cross-sectional view of the frame 101 along the frame axis 103 according to a preferred embodiment of the present invention is illustrated. The frame 101 of structural panel 100 is configured to receive at least opposite ends of the beams 105. The end members 121 are recessed or cut out along their respective lengths in order to provide a beam receiving surface 123 for receiving opposite ends of the beam 105. The recess 125 may be of any shape known to those skilled in the art (e.g. flat, bevelled or curved) that defines a beam receiving surface 123. An alternate embodiment of the end members 121 is illustrated in FIG. 10(b). In this embodiment, the end members 121 are not cut out, but rather, the end members 121 have at least one hole or passage 127 passing through the end members 121. A fastener 129 passes through the respective end members and secures the beams 105 to the frame 101.

[0055] In a preferred embodiment, the frame 101 has longitudinally extending edge flanges 131 that are generally parallel to the frame axis 103.

[0056] Referring to FIG. 16, a perspective cross-sectional view of the structural panel 100 is illustrated. The frame 101 preferably has a plurality of spars 133 extending across the frame between the edge flanges 131 and the spars 133 have sockets 135 formed therein for receiving the opposite ends of the beam elements 105.

[0057] Referring to FIG. 11, an end section 137 of the beam 105 according to a preferred embodiment of the present invention is illustrated. The beam 105 is wrapped in a beam wrapping 109 comprised of reinforcing glassy fibres 139. In a preferred embodiment the glassy fibre is standard fibreglass as sold by Dow Corning of Corning, N.Y. In an alternate embodiment, the glassy fibre is basalt based.

[0058] In a preferred embodiment, the beam wrapping 109 is a woven mat having about the same quantity of fibres 139 extending in each of two generally orthogonal directions (i.e., approximately 50% of the fibres 139 running in each direction). This is achieved by wrapping the beam wrapping 109 on a bias relative to the beam axis 107.

[0059] The beam 105 is further wrapped with wire strands 111. In a preferred embodiment, the wire strands 111 are incorporated in the beam wrapping 109. Alternately, the wire strands 111 may be an element of an additional wire strand wrap 141, which is applied over top the beam wrapping 109. In either case, the wire strands 111 run generally parallel to the beam axis 107 (FIG. 14).

[0060] The wire strands 111 are comprised of steel. In a preferred embodiment, the wires 111 are comprised of high tensile strength steel.

[0061] Referring to FIGS. 8(b) and 12, a beam end portion 137 is illustrated according to a preferred embodiment of the present invention. The beams 105 are comprised of wood, with each of the beams being made up of a plurality of laminae 143 having adjacent faces 145 laminated together and having a grain 147 running along the beam axis 107. In a preferred embodiment, the laminae 143 are aligned with the adjacent faces 145 generally parallel to the upper face 149 of the panel 100.

[0062] In a preferred embodiment, the wood laminate beam 105 is comprised of kiln dried spruce, which provides a high tensile strength per unit weight. The wood laminae 143 are dried to approximately 16% moisture or less and fixed to each other using an adhesive. In a preferred embodiment, the adhesive is manufactured by Borden Chemical Ltd. of Montreal, Canada, although any adhesive known to those skilled in the art that is as strong as or stronger than the wood may be employed.

[0063] In an alternate embodiment, the panel 100 may be used in low stress or low load bearing applications, such as a wall panel. The load bearing capabilities and weight of a panel 100 that incorporates wood laminate beams may be neither required, nor desired. The beams 105 may be comprised of a low density material, such as a foam material, which is both light weight and able to bear a load. Any low density material that can bear a load, which is known to those skilled in the art, may be employed.

[0064] Referring to FIGS. 8(a) and (b), the frame 101, beam wrapping 109, beams 105 and wire strands 111 are further wrapped and surrounded by an outer layer of reinforcing fibres 113 comprised of mat reinforcing fibres 115, thereby creating a wrapped structure. In a preferred embodiment, the reinforcing fibre 115 is a glassy fibre, such as a standard fibre glass as sold by Dow Corning of Corning, N.Y. In an alternate embodiment, the glassy fibre is basalt based.

[0065] In a preferred embodiment, the outer fibre wrap 113 has about 90% of its fibres extending longitudinally and generally parallel to the frame axis 103. The relative ratio of the fibres 115 as well as the relative directions in which the fibres 115 extend may vary with the particular use to which the structural member 100 is put and the direction and magnitude of the resultant forces exerted on the structural panel 100. For example, if the structural panel 100 is used as a roof panel, the fibres 115 may extend equally in all directions. If the panel 100 is employed as a bridge deck, then approximately 80% of the fibres 115 extend longitudinally (generally parallel with the frame axis 103) and approximately 20% of the fibres 115 run generally orthogonal to the longitudinally extending fibres 115.

[0066] After the outer fibre mat 113 is applied, the wrapped structure is permeated with and encased in an epoxy resin that extends throughout the frame 101, beam 10 wrapping 109, beams 105, wire strands 111 and outer wrap 113. The epoxy resin impregnates the fibrous material and surrounds the frame 101 and plurality of beams 105, ultimately undergoing a physical state transformation from a low viscosity fluid to a rigid solid state and thereby acting as a binding matrix of the fibrous material (109, 113), frame 101 and plurality of beams 105 to create a final composite material. In a preferred embodiment, the epoxy has exceptional adherence (without the use of special primers) to steel, such as the Jeffco 4101-08 epoxy resin and Jeffco 4101-18 epoxy hardener as manufactured by Jeffco Ltd. of San Diego, Calif.

[0067] A vacuum assisted resin transfer method (V.A.R.T.) may be used to encase the wrapped structure in epoxy resin, although any other method of impregnating and encasing a wrapped structure that is known to those skilled in the art may be used. The wire strands 111 act as a flow medium for the epoxy resin, thereby ensuring that the resin sufficiently permeates and surrounds the wrapped structure. The wire strands' 111 function as a flow medium for the epoxy resin also obviates the need to cut flow channels into the wrapped structure or any of its components.

[0068] The components of the panel 100 (i.e., the frame 101, beams 105, wraps 109 and 113, and epoxy 117) are selected to have physical properties that do not vary to such a degree that separation of the components of the composite panel occurs when the panel 100 is in use; i.e., tensile and compressive forces exerted on the panel 100 are distributed over the whole of the panel and no one component of the panel 100 bears a disproportionate degree of load bearing stress, which would result in separation of the panel 100 components.

[0069] Sample Applications

[0070] The structural panel 100 may be used in many different applications, such as building panels, piles, floor panels, wall panels, roof panels, box culverts and retaining walls. Referring to FIG. 13, a preferred application of the structural panel 100 as a bridge deck 500 is illustrated.

[0071] The bridge deck 500 has an upper face 501 that is coated in a road surfacing material 503. In a preferred embodiment, the coating 503 is a latex asphalt, such as manufactured by TJ Pounder Inc. of Brampton, Ontario, Canada. Latex asphalt is preferred because it can be applied cold, thereby obviating the need to apply a hot asphalt mixture, which may potentially compromise the structural integrity of the structural panel 100. A primer is applied first to the upper surface and then an approximately one-half inch (approximately 1.25 cm) layer of latex asphalt is applied. The opposite face or underside 505 of the deck 500 is painted with UV stabilised polyurethane.

[0072] The upper face 501 of the deck 500 is preferably canted with a longitudinally extending centre section 507 higher than and sloping toward opposite side edges 509 of the deck 500 to promote drainage.

[0073] In a preferred embodiment, the deck 500 is thicker at the opposite side edges 509 than at the centre section 507 thereby providing a curb 511 at the side edges 509. Having a deck 500 with curbs 511 at the opposite side edges 509 results in overall enhanced stiffness of the deck 500, thereby allowing it to be thinner at the centre section 507 than would be required of a panel having the same load bearing capacity but of generally constant thickness.

[0074] The present invention is defined by the claims appended hereto, with the foregoing description being illustrative of the preferred embodiments of the invention. Those of ordinary skill may envisage certain additions, deletions and/or modifications to the described embodiments, which, although not explicitly suggested herein, do not depart from the scope of the invention, as defined by the appended claims. For example, the beams may be of non rectangular cross-sectional configuration (such as circular, elliptical, triangular etc.) and the beams need not have their respective longitudinal axes coplanar. Furthermore in some applications curved rather than straight beams may be desirable. 

We claim:
 1. A structural panel comprising: a frame having a longitudinal frame axis; a plurality of beam elements mounted in said frame, each of said beam elements having a respective longitudinal beam axis generally parallel to said frame axis; a reinforcing fibre wrapping extending around each of said beam elements, said wrapping having a fibre direction running generally obliquely to said beam axis; a plurality of wire strands extending about said wrapping, said wire strands running generally parallel to said beam axis and said frame axis; an outer layer of reinforcing fibre mat surrounding said frame, said wrapped beams and said wire strands, said mat having a majority of its fibres running generally parallel to said frame axis; and, a solidified epoxy resin extending throughout and encasing said beam elements, beam wrapping, wire strands, outer layer and frame.
 2. The structural panel of claim 1 wherein: said frame is configured to receive at least opposite ends of said beams; and, said reinforcing fibre mats of said beam wrapping and said outer layer are of a glassy fibre.
 3. The structural panel of claim 2 wherein: said reinforcing fibre mat of said beam wrapping is a woven mat having about the same quantity of fibres extending in each of two generally orthogonal directions; and, said reinforcing fibre mat of said outer layer has about 90% of its fibres extending longitudinally generally parallel to said beam axis.
 4. The structural panel of claim 3 wherein: said beams are of wood; and, said wire strands are of steel.
 5. The structural panel of claim 4 wherein: each of said beams is made up of a plurality of panels having adjacent faces laminated together and having a grain running along said beam axis; and, said wires are of high tensile strength steel.
 6. The structural panel of claim 5 wherein: said structural panel is a bridge deck having an upper face coated with a road surfacing material; and, said planks are aligned with said adjacent faces generally parallel to said upper face.
 7. The structural panel of claim 6 wherein: said upper face is canted with a longitudinally extending centre section higher than and sloping toward opposite side edges of said structural panel to promote drainage.
 8. The structural panel of claim 7 wherein: said structural panel is thicker at said opposite side edges than at said centre section thereby providing a curb at said outer edges with enhanced stiffness to allow said structural panel to be thinner at said centre section than would be required of a panel having the same load bearing capacity but of generally constant thickness.
 9. The structural panel of claim 8 wherein: said frame has longitudinally extending edge flanges generally parallel to said frame axis; said frame has a plurality of spars extending there across between said edge flanges; said spars have sockets formed therein toward said upper face for receiving said opposite ends of said beam elements. 